Synchronization of the horizontal and vertical scans of a television receiver may be accomplished by triggering the deflection circuits directly by incoming horiziontal and vertical synchronizing (sync) pulses derived from the composite video signal. Under some conditions, such as when the receiver signal is weak or there is considerable signal noise interference, the deflection circuits may fail to trigger on the proper synchronizing pulse, resulting in loss of receiver synchronization.
To provide more reliable synchronization, circuits were developed which produced horizontal and vertical rate pulses by frequency dividing a high frequency clock signal. The derived horizontal rate signal is locked to incoming horizontal sync pulses by phase lock loop circuitry to provide accurately timed synchronization of the horizontal deflection circuit.
The internally generated vertical rate pulses may also be used to provide accurate triggering of the vertical deflection circuit. The external incoming vertical sync pulses normally occur in coincidence with the internally generated vertical rate pulses, so that the internal pulses may be used to synchronize the deflection circuit. A vertical sync window having a selected time interval is chosen, during which coincidence between the internal and external pulses is expected. If coincidence is not detected for a predetermined number of fields, it may be desirable to provide synchronization by the external pulses directly.
Receiver synchronization may be adversely affected when the video source is prerecorded material from a video tape recorder. Two recorder playback heads are typically positioned on opposite sides of a rotating headwheel such that the heads alternately scan one field of information diagonally across the tape. Switching of the heads after each field causes signal transients which are desirably timed to occur just before the vertical blanking interval so that any video disruptions go unnoticed. However, head switching may cause significant disruption of the horizontal synchronizing information. If the horizontal synchronizing frequency and phase control loops respond too slowly, video displacement may be noticeable at the top of the kinescope display screen for the first several horizontal scan lines. It is possible to increase the phase control loop gain in timed coincidence with the vertical blanking interval in order to decrease the horizontal sync phase shift recovery time when using a tape recorder so that substantially all phase correction occurs during the vertical blanking interval. Although the loop gain will be increased during each vertical blanking interval regardless of the receiver program source, this feature will only be advantageously realized when using a video tape recorder.
Another problem which may occur with video tape recorders results from the use of rapid forward and reverse search modes. To provide a picture during rapid scan operation, some video information is deleted. This may be accomplished by deleting entire horizontal scan lines. Receivers which utilize internally produced vertical sync pulses, such as previously described, typically generate those sync pulses based on the known numerical relationship of the number of horizontal scan lines per field. Changes in this numerical relationship, such as occur during the rapid search mode of video tape recorders, may cause the external vertical sync pulses recorded on the tape to occur outside the internal pulse coincidence window interval, resulting in improper vertical synchronization. This problem may be corrected by enlarging the vertical sync window interval to insure that the external pulses occur within the window interval. During viewing of normal broadcast television signals, however, this may reduce vertical sync noise immunity. It is therefore desirable to provide increased horizontal phase lock loop gain and an enlarged vertical sync window only during the use of a video tape recorder.
Material recorded on video discs may also cause receiver synchronization problems. Disc eccentricity or speed variations in the master cutting stylus may cause variations in horizontal sync timing. In order to minimize any visible effect that may occur, it is desirable to increase the horizontal phase control loop gain which increases the loop response rate. However, this may increase picture jitter when receiving ordinary broadcast signals.
The previously described problems associated with video tape recorders and video disc players are effectively controlled by providing a receiver having a horizontal phase control loop with a fast response, and a large vertical sync coincidence window interval. These characteristics, although desirable for video tape and disc equipment, may undesirably compromise the operation of the receiver during ordinary broadcast signal reception or during the reception of video signals from other sources of video information where other receiver operating or synchronization characteristics may be desired.